1. Field of the Invention
The present invention relates to a semiconductor light source typically represented by a laser diode and, more in particular, it relates to a semiconductor light source suitable for use in writing light sources of optical disks or magneto-optical disks, and for use in optical communication modules, optical communication systems, and optical networks.
2. Description of the Related Art
At present, laser diodes with a high degree of reliability have been demanded for use in writing to optical disks or magneto-optical disks, as well as in optical communication systems. Such laser diodes are required to operate for a long time, stably in a basic mode. A coating film made of an insulator film is applied to the surface of a semiconductor forming a light emission surface of semiconductor laser diodes. This is used to improve the light outputting efficiency by obtaining an appropriate reflectivity and to prevent lowering of a maximum output due to the increase in a threshold carrier density. For the coating film, a thin silicon oxide film is used as a low reflection film on one side, and a multi-layered film comprising a thin silicon oxide film and a thin hydrogenated amorphous silicon film as a high reflection film formed on the other side of an optical resonator. It is reported, for example, by T. Yuasa, et al. Applied Physics Letters, vol. 34, pp 685.
Further, a vacuum-through process of conducting formation of a facet and formation of a facet protection film by cleave through a super-high vacuum has been intended to suppress light absorption caused by oxidation of a facet of an optical resonator. In U.S. Pat. No. 5,063,173, for example, a facet of an optical resonator is formed by cleaving a laser diode wafer in a super-high vacuum, and then directly depositing a thin film comprising Si, thereby forming an oxide film on a semiconductor surface in the facet of an optical resonator.
In a case of using aluminum oxide for the facet protection film, since defects, if any, caused by lack of oxygen in an aluminum oxide film results in a problem in view of the resistance to damp of the film, Japanese Patent Laid-open No. 2003-86864 discloses that a resistance to damp can be improved by depositing molecules of water to the surface of aluminum oxide to compensate the defects. Further, Japanese Patent Laid-open No. 2002-223026 discloses that the reliability of the laser diode can be improved by lowering the internal stress of the facet coating film of the laser device.
It is necessary for the laser diode with a high degree of reliability to suppress facet deterioration caused by the laser driving operation. The facet deterioration is caused by repeating of mechanisms as follows: the facet temperature is elevated by absorption of a portion of light oscillated by laser driving operation to the non-radiative recombination center at the facet, and the non-radiative recombination center is thereby multiplied to further raise the facet temperature. Most of the non-radiative recombination centers are formed by the trap level in the forbidden band formed by diffusion of oxygen in the semiconductor. Accordingly, to minimize the facet deterioration, it is essential to minimize diffusion of oxygen to the semiconductor facet.
The facet deterioration is remarkable in the laser diode of a structure containing aluminum in an active layer. The unique study of the present inventors has found that in a case where aluminum is contained in an active layer, when an amorphous silicon film or micro-crystal silicon film is formed adjacent to the semiconductor, bending of a band in which the band of the semiconductor is in an electron accumulation state is caused near the boundary, and the leak current near the boundary increases to promote the deterioration even when there is no oxygen diffusion.
Further, in a case where a silicon oxide film or a silicon nitride film is deposited adjacent to the semiconductor facet, since aluminum in the semiconductor tends to be bonded with oxygen more easily than with silicon, oxygen in the film diffuses into the semiconductor causing bonding with aluminum to promote deterioration. Also in a case of using the silicon nitride film, since a small amount of oxygen contained in the film is bonded preferentially with aluminum, no suppression effect can be obtained.
Also in a device not incorporating aluminum, arsenic in the semiconductor is bonded with oxygen to form a trap level, which causes similar deterioration.
Further, a reflection film formed to the facet often has a multi-layered structure so as to obtain a desired reflectivity. Even when the internal stress in each of the layers is small, the total stress on the semiconductor increases because of the multi-layered structure. Accordingly, since the amount of strain caused in the semiconductor crystals is increased, oxygen tends to diffuse to the boundary making it difficult to minimize deterioration on the facet.